Preparation of aromatics from a naphtha feed



R. N. SHIRAS 2,921,015

PREPARATION OF AROMATICS FROM A NAPHTHA FEED Jan. 12, 1960 Filed July29, 1957 HIS TTORNEY T in 5 m mm m N.

5 v 0Q m1 mm (0 II) I 0 1 INVENTOR mm mm on United States PatentPREPARATION OF AROMATICS FROM A NAPHTHA FEED Russell N. Shires, Oakland,Calif., assignor to Shell Development Company, New York, N.'Y., acorporation of Delaware Application July 29, 1957, Serial No. 674,789

2 Claims. (Cl. 208-79) This invention relates to an improved process:for the production and recovery of aromatic hydrocarbons from mixturesof hydrocarbons and to a method which permits the recovery of a heavyaromatic concentrate, and, ifdesired, a light aromatic concentrate.

Itis well known that aromatic hydrocarbons may be separated. fromhydrocarbon mixtures containing them by -extraction with any of severalavailable solvents. The eflicient extraction of the aromatics often isno great problem. as the solvent and conditions may be selected toobtain an extract phase containing substantially all or. only a part ofthe aromatics in the feed. A chief difliculty in. recovering aromaticsby extraction is that most solvents selective for aromatics will alsodissolve a sig-v nificant. amount of the non-aromatic material whichgives rise to a problem in the fractionation of the extract as aromaticsand non-aromatics of the same. boiling range cannot be efiicientlyseparated by the usual distillation techniques. As a result, theseparated aromatic fractions are frequently not suitable for the moreexacting needs. With the presently large requirement for concentratesfor fuel blending, there is a cleardernand for processes that willpermit a more sharp separation of aromatics and non-aromatichydrocarbons.

It is an object of the present invention to provide an improvedmethodfor the separation of aromatic hydrocarbons from non-aromatichydrocarbons and in particular for the preparation of aromaticconcentrates. Another. object of the present invention istofurnish aprocesswhich may be used for the joint production of both aheavyaromatic concentrate and a light. aromatic concentrate.

The improved process of this invention is especially suitablefor theseparation of aromatic concentrates from catalytically reformed, e.g.hydroformed, such as platformed naphtha feeds. As pointed out earlier,an acute problem exists. in separating aromatics and saturates ofthesameboiling range; however, by the practiceof the process. oftheinstant invention, the two types of compounds may be readily andefficiently segregated.

The advantages of the present invention and its prac.

tice will be better understood from the detailed description of it whichwill be made with reference to the accompanying drawing wherein the solefigure is a schematicrepresentation of a preferred system for thepractice of the improved process.

Thepresent invention provides a process in which the naphtha feed isfirst subjected to a sizing operation to divide the feed into apredominantly light saturate naphtha fraction and into a heavy naphthafraction made up principally of heavy saturates. The heavy naphthafraction is then subjected to aromatization, for example, byhydroforming over a platinum-on-alumina catalyst to increase appreciablyits heavy aromatic content and of course simultaneously to reduce itsheavy saturate content. The aromatized heavy naphtha is then passed toan ,,inter mediate section of a countercurrent extractionv zone. Aselective solvent for aromatic hydrocarbons.

which is relatively immiscible with non-aromatic hydro-. carbons isadmitted to one end of the zone in extractive contact with thearomatized heavy naphtha. vent should have a relatively selectivesolubility for the heavy aromatics at the elevated temperature of theex-. traction, preferably in excess of 300 F., and a low solubility atthe temperature of operation of the desorbing zone described infra,generally in the neighborhood of F. Since the solvent and extractedmaterial are not separated by distillation, there is greater latitude inthe selection of the. solvent because no spread in volatility isessential. This may prove to be an advantage where the naphtha beingprocessed extends into the kerosene range. Among the preferred solventsare the glycols, includingthe polyglycols, and particularly diethyleneglycol. There is formed in the extraction zone an aromatic-enrichedextract phase and a raffinate phase containing predominantly heavysaturates. The process employs a countersolvent in this first extractionzone, Which is a portion of the light naphtha from the earlier mentionednaphtha feed fractionation, to bring about a higher rejection of theheavy saturates from the extract phase. The extract phase upon itsremoval from the extraction zone is cooled to effect the formation of asolvent phase and a hydro-. carbon phase and the cooled material is thenpassed to the top of a desorbing (second solvent extraction) zone.Another portion of the light naphtha (from the naphtha feed fraction) isadmitted to the bottom of this desorbing zone to obtain a desorbedsolvent which is substantially free of heavy aromatics and a raffinatecontaining principally heavy aromatics and light naphtha. The rafiinateis fractionated into a heavy aromatic concentrate and a light naphthafraction. The light naphtha fraction may be used as a gasoline blendingstock or as a source of specialties but it is preferred to process itfurther to provide a source of light aromatics, as by catalyticreforming or to subject it to an isomerization process to produce highoctane, branched isomers.

Various ones of the common selective solvents for aromatics, e.g.phenol, nitrobenzene, the sulfolanes, aceto nitrile, furfural, or one ofthe several glycols may be used. A preferred solvent is diethyleneglycol which may be used alone or diluted with, say 2 to 10% by weightof water. Representative of others of the glycol solvents which may beutilized with somewhat less ad-.

vantageous results are ethylene glycol, triethylene glycol,

suitable sulfolanes are sulfolane itself and many of their derivativessuch as hydrocarbon-substituted sulfolanes, including alkyl sulfolanes,e.g. 3-methylsulfolane, pref erably containing not more than 14 carbonatoms; hydroxy sulfolanes such as S-sulfolanol, 3-methyl-4-sulfolanol,etc. sulfolanyl others such as methyl-3-sulfolanyl ether; and sulfolanylesters such as 3-sulfolanyl acetate.

It is preferred that the naphtha feed (before catalytically reforming)be debutanized and have a boiling range within about 60 F. to 400 F. Thefeed is preferably divided into a heavy naphtha fraction containingprim-- cipally the C and heavier saturates and the C and heavieraromatics and into a light naphtha fraction made up principally of the Cand lighter saturates and C and C aromatics. The C saturate hydrocarbonswill preferably be split between the two fractions and generally thenormal C saturates will be found in the heavy naphtha fraction and thebranched C saturates in the light naphtha fraction.

The aromatization of the heavy naphtha feed may be had by catalyticallyreforming, for example, by either hydroforming or platforming inaccordance with standard operating conditions.

matic, say 60% aromatic and 40% saturates. It is e'ssen- The 501-.

It is preferred that the reformed material should have a compositionwhich is chiefly aro ran'e as the aromatics of the reforrnate be removedtherefrom. This removal is facilitated in the extraction zone by the useof a portion of the light naphtha (from the naphtha feed fractionation)as countersolvent. In such usage the light saturates of thecountersolvent tend to displace the heavy saturates dissolved in theextract phase. This supplanting makes for better separation of thesaturates and the aromatics in the subsequent fractionation procedure.The separation afforded by simple solvent extracting in the absence ofthe light saturate countersolvent is not adequate for a sharp subsequentfractionation of saturates and aromatics. The rafiinate from theextraction zone, which contains chiefly heavy saturates, may be recycledin part or wholly to the aromatization step. The extract is strippedfrom the solvent by a desorbing procedure promoted by cooling of theextract phase. Preferably, the extract phase out of the extraction zoneis passed in indirect heat exchange with desorbed solvent from thedesorbing zone. This procedure assists in the needed cooling of theextract phase and simultaneously raises the temperature of the recycledsolvent which is introduced to the extraction zone at an elevatedtemperature. The extract phase in the desorbing zone is counterextracted with an additional portion of the light naphtha to assist inthe stripping of the solvent. It may be desirable in some instances tostrip at least a portion of the solvent (from the desorber) with aquantity of heavy naphtha which may then be recycled to form a portionof the naphtha feed. This additional stripping of the desorbed solventwill lower still further its light saturate content.

The rafiinate from the extraction zone is preferably water-washed torecover the solvent dissolved therein and the heavy aromatic and lightnaphtha raflinate' from the desorbing zone is likewise water-washed toreduce solvent loss. The latter water-washed raflinate is passed to aconventional fractionating tower where it is readily separated into anoverhead light naphtha fraction and into a heavy aromatic concentrate.The heavy aromatic concentrate will generally constitute approximatelyan 85% recovery of the aromatics from the reforming (aromatization). Thesharp separation of saturates and aromatics is made possible because ofthe earlier processing steps devoted to the removal of heavy saturatesfrom the aromatic stream. i

In the following description of a system for performing the improvedprocess of the invention, the solvent used is diethylene glycol which isthe preferred material; it is to be understood, however, that others ofthe common high boiling aromatic solvents may be utilized, in whichevent the conditions of operation will be suitably adjusted.

Referring to the drawing, a debutanized naphtha feed (having preferablyabout a 60-390 F. boiling range) of say 1,000 barrels per day isintroduced through a line to a central section of a fractionating toweror naphtha splitter 11. The bottom of the tower is preferably maintainedat a temperature suitable to give a bottom product having a boilingrange of approximately 250.-390 F.

while the top of the tower is operated to provide a dis-.

tillate with the boiling range of approximately 60250 F. The naphthafeed is split into a heavy naphtha fraction of say 440 barrels per dayand a light naphtha fraction of 560 barrels per day. The proportions ofthe light and heavy fractions may be varied somewhat but generally asplit providing 40 to 60% heavy naphtha gives best results.

. The heavy naphtha fraction is passed by a line 12 to a catalyticreforming unit 14 where it is subjected to a conventional reformingoperation to increase significantly its aromaticcontent to say 60%. Thereformate is removed from the reformer through a line 15 which opensinto a central portion of a reformate extractor 17. This extractor isoperated at an elevated temperature and when diethylene glycol isemployed as a solvent, a suitable temperature is about 350375 F. Thesolvent enters the extraction zone at its top through a conduit 18, fromwhence it passes in counterflow to the reformate, dissolving a principalportion of the heavy aromatics produced in the reformer. The heavysaturates of the reformate pass upwardly through the extractor and leavein the rafiinate phase through a conduit 20. A portion of the lightnaphtha fraction from the splitter 11, say about 124 barrelsper day isintroduced by a line 13 to the base of the extractor as a countersolvent, i.e. a solvent selective for paraffin hydrocarbons and at leastpartially immiscible with the polar solvent. The light naphthasubstantially supplants the heavy saturates that are contained in theextract phase.

Theraffinate from the extractor will contain princi pally heavysaturates, a small amount of heavy aromatics and some light naphtha. Ina typical operation based on 1,000 barrels per day of naphtha feed (andwhich employs a moderate recycling of of the rafiinate to theplatformer) the rafiinate may be expected to comprise approximately 159barrels per day of heavy saturates, 32 barrels per day of heavyaromatics and 44 barrels per day of light naphtha. The raffinate iswaterwashed in a tower 22 to remove therefrom the small amount ofsolvent that it contains and leaves that tower via line 92 whichbranches into lines 93 and 94. At times it may be desirable to recycle asubstantial quantity of the heavy saturate raflinate via the line 93back to the reformer. In other instances it may be desirable to transferall or a portion of the rafilnate to a product tank 95. The wash waterleaves the washing tower 22 through a line 24 and is introduced to thebottom portion of a water solvent fractionating tower 26 via a line 27.Water is removed overhead by a line 29 and the recovered solvent isreturned via line 30 to the solvent system. The extract phase exits fromthe bottom of the extractor in a line 32 which first opens into a heatexchanger 33 and from the latter unit, the still warm extract phasepasses to a water cooler 34, from whence it is moved through line 36 toa top portion of a desorber tower 37. A typical composition of theextract phase (again based on 1,000 barrels of naphtha feed daily) willbe about 120 barrels of light naphtha, l1 barrels of heavy saturates,216 barrels of heavy aromatics and approxivfor aromatics is quitetemperature dependent and with this appreciable drop in temperature, theextract phase forms asolvent phase and a hydrocarbon phase. Theintroduced cooled material to the desorber tower forms an overlyinghydrocarbon layer which is removed from the tower through a line 39. Thedesorbed solvent leaves the tower at its base through a line 41 whichcarries the cool solvent through the aforementioned heat exchanger 33where its temperature is raised to the neighborhood of 350 F. and fromwhence it passes through a second heat exchanger 43, steam heated, whereits temperature is further elevated to the operating temperature of say375 F. of the extractor tower. A second portion of the light naphthafraction from the naphtha splitter 11 is transferred through a line 45to a bottom section of the desorber tower 37. This material which isintroduced in an appreciable quantity, say 240 barrels per day (againbased on 1,000 barrels per day naphtha feed) passes countercurrently tothe downwardly descending solvent within the desorber. The light naphthamaterial acts as a desorbent and assists in the stripping of thedissolved aromatic hydrocarbons from the solvent.

, V In some iii'stancesin'order"to loweithe lightnaphtha loss tojtlie"heavy" saturate raffinate 'of -the extractor 17, itwill 'lieeoniedesirable to'pa's's at" leastf'a portion of the desorbe'd solyent'fromthe tower 37 through a line 47 to the 16p of awash tower 49- where thesolvent is passed in" counter-current flow to "a"heavy-saturate streamintroduced'to thebase of that'towervia a line 50. The heavy naphthaemployed" heremaybe a portion of the heavy naphtha from the splitter 11or it may be a portion of the heavy saturate recycled from the raflinatewash tower 22 in which event it is transferred to line 50 via line 51which is a continuation of the heavy recycle line 93. Thiscountercurrent washing of the solvent with the heavy naphtha will lowerstill further the hydrocarbon content of the solvent. The strippedsolvent exits from the base of the hydrocarbon wash tower 49 through aline 52 which opens into the earlier mentioned line 41 from the desorberat a point immediately preceding the heat exchanger 33. The hydrocarbonwash materialis removed from the tower 49 by a line 54 to the base of awater wash tower 56. The purpose of this latter tower is to recover anysolvent that may have been carried over with the hydrocarbon stream fromthe preceding tower 49. The wash water for this purpose is admitted tothe top of the tower 56 by a line 58. The solvent-containing wash wateris transferred via a line 59 to the earlier mentioned water-solventfractionating tower 26 (or to a separate fractionating tower). Thewaterwashed heavy aromatic material from tower 56 is removed by a line97 and added to the naphtha feed in line at a point preceding thesplitter 11. The heavy naphtha material is recycled in this fashionsince it contains a portion of light naphtha and should consequently beresplit before undergoing further processing.

The raflinate contained in the line 39 is transferred to the base of awaterwash tower 61 where it is passed in 'countercurrent flow todownwardly descending water, extracting any solvent that may be presentand is removed through a line 63 from the top of the tower. On a dailybasis of this example the rafiinate is made up of approximately 208barrels of heavy aromatic and 300 barrels of light naphtha. These twoprincipal components of the raffinate have significantly differentboiling ranges which difference permits an effective and sharpfractionation of the ralfinate in a distillation column 66 which isdesigned and operated under conditions to achieve recovery of about 98%of heavy aromatics in the feed to this column 66 and approximately a 93%rejection of the light naphthas of the column feed. For example, thecolumn may have say 60 bubble trays and be operated with from 2 to 2.5moles of reflux per mole of column feed. The heavy aromatic concentratewhich is removed to storage 68 as a liquid from the base of the towerthrough a line 67 will generally represent an overall recovery ofapproximately 85% of the heavy aromatics originally contained in thereformate.

The light naphtha feed leaves the tower through a line 69 which combineswith a line 70 from the top of the naphtha feed splitter 11 to form aline 71. The line 70 transfers the excess light naphtha which has notbeen utilized either as counter solvent in the extractor 17 or asdesorbant in the desorbing tower 37. The quantity of 7 flow through theexcess light naphtha line 70 will vary from time to time and there mayeven be instances where no material is transferred therethroughdepending on how the naphtha feed is split and the proportions of lightnaphtha used as counter solvent and desorbant. The light naphtha carriedby the combined line 71 may be transferred to a storage tank 73 fromwhich it may be removed as needed for use, say as a gasoline blendingstock or as a specialty source. In an alternative the material may befurther processed in a separate light aromatic synthesis plant which mayor may not involve 6 naphtha isbmerization andmay be'designed' for the"pro= duction of nitration grade'benzeneand/or toluene.

In' the'drawing" the light naphtha of line 71 is passedto anaromatizatio'n unit 75 where the material is sub-' jected toaconventional processing. to furnish a catalytically reformed materialcontaining a large quantity of light aromatics. The'aromatized materialflows through a line 76 to a solventextractor 77 where it is contactedwith a solventgpreferably the same solvent employed for extracting. theheavy. naphtha. This solvent enters the extractor adjacent it s topthrough a line 78. The various solvents and conditions discussed earlierin connection with the other extraction may be employed here and againthe solvent should be thermally stable as the extraction is preferablycarried on atan elevated temperature. The extract phase leaves from thebase of the tower via conduit 79 and is passed through the heatexchanger 81 against the lean solvent carried by the line 78. Theextract phase is introduced to a central section of a solvent strippingtower 83. Heat is supplied to the stripping section at the base of thetower through a reboiler 84. A light aromatic stream is removed at anintermediate point of the tower through a line 85 and passed to storage95. A gaseous stream containing a higher concentration of lightsaturates is removed from the top of the stripping tower through a line87 and returned to the base of the solvent extractor 77 where it servesas a back wash, passing in countercurrent flow to the downwardlydescending extract phase. Reflux for the separations made in tower 83 isprovided by the condenser 86. The temperature of operation of the tower83 and amount of reflux will depend chiefly on the particular solventemployed and the composition of the extract. A light saturate rafiinateis transferred from the top of the extraction tower 77 in a conduit 88to a water washing tower 89 where it is passed in countercurrent flow towater introduced through a line 90. Solvent containing wash water istransferred from the Wash tower by a line 91 to the previously describedWater solvent distillation column 26. The light saturate stream out ofthe wash tower 89 may be recycled in part as a portion of the feed tothe reformer 75 through a line 93 and the line 71 or it may be moved inpart or in whole via a line 94 to storage or further processing.

1 claim as my invention:

1'. A process for the production of aromatics from a naphtha feed whichcomprises dividing the naphtha feed into a light naphtha fractioncontaining predominantly light saturates and into a heavy naphthafraction containing predominantly heavy saturates; subjecting the heavynaphtha to aromatization, thereby increasing appreciably its heavyaromatic content and reducing its heavy saturate content; introducingthe aromatized heavy naphtha into a counter flow extractive zone at anintermediate point thereof; admitting a polar solvent selective foraromatics and relatively immiscible with non-aromatic hydrocarbons intoone end of the zone in extractive contact with the aromatized heavynaphtha to form an aromatic-enriched extract phase and a rafiinate phasecontaining predominantly heavy saturates and introducing to the otherend of the extraction zone a counter solvent comprising a portion ofsaid light naphtha, thus effecting a higher rejection of the heavysaturates from the extract phase; withdrawing the extract phase fromsaid other end of the zone, cooling said extract phase to effect theformation of a solvent phase and a hydrocarbon phase, and passing thecooled material to a desorbing zone; sweeping the desorbing zone withanother portion of said light naphtha to obtain a solvent substantiallyfree of the heavy aromatic and a raffinate containing heavier aromaticsand light naphtha; fractionating the latter rafl'lnate into a heavyaromatic concentrate and a light naphtha fraction; subjecting the lightnaphtha fraction to an aromatization operation, thereby providing areformed light naphtha rich in light aromatics; extracting the lightaremade-enriched reformed material :with a high boiling preferentialsolvent for aromatics to obtain a light saturate-enriched rafiinate andan aromatic-enriched extract phase; and passing the latter extract phaseto a fractionation zone and there recovering alight aromaticconcentrate.

2. A process in accordance with claim 1 wherein at least a portion ofthe light saturate rafiinate is recycled to said light naphthaaromatization operation.

References Cited in the file of this patent UNITED STATES PATENTS2,176,396 Fenske et al., Oct. 17, 1939 8 Haensel et al. Nov. 17, 1953Haensel Jan. 4, 1955 Shelton et al Nov. 22, 1955 Georgian Dec; 20, 1955Evans Nov.6 1956 Grote L. Nov. 13, 1956 Kassel et al. June 10, 1958Haensel Sept. 23,1958

1. A PROCESS FOR THE PRODUCTION OF AROMATICS FROM A NAPHTHA FEED WHICHCOMPRISES DIVIDING THE NAPHTHA FEED INTO A LIGHT NAPHTHA FRACTIONCONTAINING PREDOMINANTLY LIGHT SATURATES AND INTO A HEAVY NAPHTHAFRACTION CONTAINING PREDOMINANTLY HEAVY SATURATES, SUBJECTING THE HEAVYNAPHTHA TO AROMATIZATION, THEREBY INCREASING APPRECIABLY ITS HEAVYAROMATIC CONTENTENT AND REDUCING ITS HEAVY SATURATE CONTENT, INTRODUCINGTHE AROMATIZED HEAVY NAPHTHA INTO A COUNTER FLOW EXTRACTIVE ZONE AT ANINTERMEDIATE POINT THEREOF, ADMITTING A POLAR SOLVENT SELECTIVE FORAROMATICS AND RELATIVELY IMMISCIBLE WITH NON-AROMATIC HYDROCARBONS INTOONE END OF THE ZONE IN EXTRACTIVE CONTACT WITH THE AROMATIZED HEAVYNAPHTHA TO FORM AN AROMATIC-ENRICHED EXTRACT PHASE AND A RAFFINATE PHASECONTAINING PREDOMINANTLY HEAVY SATURATES AND INTRODUCING TO THE OTHEREND OF THE EXTRACTION ZONE A COUNTER SOLVENT COMPRISING A PORTION OFSAID LIGHT NAPHTHA, THUS EFFECTING A HIGHER REJECTION OF THE HEAVYSATURATES FROM THE EXTRACT PHASE, WITHDRAWING THE EXTRACT PHASE FROMSAID OTHER END OF THE ZONE, COOLING SAID EXTRACT PHASE TO EFFECT THEFORMATION OF A SOLVENT PHASE AND A HYDROCARBON PHASE, AND PASSING THECOOLED MATERIAL TO A DESORBING ZONE, SWEEPING THE DESORBING ZONE WITHANOTHER PORTION OF SAID LIGHT NAPHTHA TO OBTAIN A SOLVENT SUBSTANTIALLYFREE OF THE HEAVY AROMATIC AND A RAFFINATE CONTAINING HEAVIER AROMATICSAND LIGHT NAPHTHA, FRACTIONATING THE LATTER RAFFINATE INTO A HEAVYAROMATIC CONCENTRATE AND A LIGHT NAPHTHA FRACTION, SUBJECTING THE LIGHTNAPHTHA FRACTION TO AN AROMATIZATION OPERATION, THEREBY PROVIDING AREFORMED LIGHT NAPHTHA RICH IN LIGHT AROMATICS, EXTRACTING THE LIGHTAROMATIC-ENRICHED REFORMED MATERIAL WITH A HIGH BOILING PREFERENTIALSOLVENT FOR AROMATICS TO OBTAIN A LIGHT SATURATE-ENRICHED RAFFINATE ANDAN AROMATIC-ENRICHED EXTRACT PHASE, AND PASSING THE LATTER EXTRACT PHASETO A FRACTIONATION ZONE AND THERE RECOVERING A LIGHT AROMATICCONCENTRATE.